1. Field of the Invention
The present invention relates to HC (hydrocarbon) adsorbing materials and devices for judging deterioration of such materials. More particularly, the present invention relates to techniques for judging deterioration of a HC adsorbing material using a zeolite.
2. Description of the Related Art
An exhaust gas-purifying catalyst arranged in the exhaust passage of an internal combustion engine, such as a three-way catalyst, is constantly exposed to high-temperature exhaust gas and thus is liable to deteriorate with age under the influence of the exhaust heat. It is therefore important to keep track of deterioration of the exhaust gas-purifying catalyst.
Generally, deterioration of the exhaust gas-purifying catalyst is judged on the basis of a correlation between oxygen storage capacity of an oxygen storage material (OSC material) contained in the exhaust gas-purifying catalyst and deterioration of the catalyst. Specifically, an oxygen sensor is arranged downstream of the exhaust gas-purifying catalyst, for example, and if the oxygen concentration on the downstream side of the exhaust gas-purifying catalyst, detected by the oxygen sensor, approaches the oxygen concentration on the upstream side of the exhaust gas-purifying catalyst, it is judged that the oxygen storage capacity of the oxygen storage material has lowered, that is, the exhaust gas-purifying catalyst has deteriorated.
Meanwhile, HC adsorbing materials capable of adsorbing HC in exhaust gas have recently been developed, and zeolites are commonly known as such HC adsorbing materials. However, the HC adsorbing materials also are liable to deteriorate with age under the influence of exhaust heat, and therefore, it is still important to keep track of deterioration of the exhaust gas-purifying catalyst.
An exhaust gas-purifying catalytic converter has also been developed in which a three-way catalyst and a HC adsorbing material are formed in layers, for example. In such a catalytic converter, there is a correlation between deterioration of the HC adsorbing material and that of the three-way catalyst, and the correlation is utilized to estimate deterioration of the HC adsorbing material through the detection of deterioration of the three-way catalyst.
The HC adsorbing material has the property of adsorbing HC at low temperatures, as in a cold state of an internal combustion engine, and desorbing the adsorbed HC at high temperatures, as in a warm state of the engine. A deterioration diagnosis technique utilizing this property of the HC adsorbing material has also been developed in which O2 sensors are arranged upstream and downstream of the HC adsorbing material, respectively, and deterioration of the HC adsorbing material is judged from the output characteristic of the downstream O2 sensor observed during the warm-up of the engine (cf. Unexamined Japanese Patent Publication No. H09-79028).
In the method wherein deterioration of the HC adsorbing material is estimated by judging deterioration of the three-way catalyst, however, the correlation between deterioration of the HC adsorbing material and that of the three-way catalyst needs to be obtained in advance. Further, deterioration of the HC adsorbing material cannot be judged in the case where the HC adsorbing material alone is used, namely, in the absence of the three-way catalyst.
Also, according to the technique disclosed in the aforementioned Japanese patent publication, the output of the downstream O2 sensor is continuously monitored during the warm-up of the engine, to diagnose the HC adsorption/desorption characteristic of the HC adsorbing material. Thus, if, during the warm-up, the engine operating condition fluctuates with respect to a reference operating condition, the accuracy of judgment is likely to lower to an undesirable level. Moreover, the technique is associated with the problem that deterioration of the HC adsorbing material can be judged only at the time of cold start of the engine.